Since synthetic polymers, including poly-ethylene, -styrene, and -propylene, must retain their functionality over widely varying conditions of temperature and pressure fluctuation, flame exposure, etc., they are synthesized to have backbones of only carbon atoms, which makes them resistant to chemical and enzymatic degradation. The introduction of heteroatoms into the polymer backbone creates functional groups, such as esters and amines, which increases the polyester's susceptibility to hydrolytic cleavage, that is, degradation, thereby improving the polyester's ability to biodegrade upon disposal.
Polyester polymers are susceptible to hydrolytic cleavage by either chemical or enzymatic treatment. Many polyesters, especially those made up solely of aliphatic monomers, e.g. polyhydroxybutyrate and poly(ε-caprolactone), are considered biodegradable. See Muller et al. (2001) J. Biotechnol., 86:87–95; Abou-Zeid et al. (2001) J. Biotechnol., 86:113–126. Aliphatic polyesters, however, lack desired material properties, especially durability, for many applications because of their low melting temperatures and increased susceptibility to degradation. In contrast, aromatic polyesters, which are exemplified by polyethylene terephthalate (PET), have the desired durability for use in a wide array of applications but are generally considered non-biodegradable. See Kint, D. and Munoz-Guerra, S., (1999) Polym Int 48:346–352. The tension between durability and biodegradability is an ever-present consideration in achieving cost-effective waste disposal. On the one hand, throwaway goods made of aliphatic polyesters are environmentally attractive yet lack acceptable durability. On the other hand, goods made of aromatic polyesters have the preferred sturdiness but their disposal is much more ecologically burdensome.
A partial resolution to this dilemma is the use of aliphatic-aromatic co-polyesters which yield durable and biodegradable products. However, the use of high aromatic content co-polyesters in throwaway goods is still not entirely satisfactory as the rate of biodegradation is proportional to the content of aromatic acid in the co-polyester. In essence, the tension between durability and biodegradability still remains: the greater the aliphatic content, the more biodegradable but less durable the good. And, the greater the aromatic content, the greater the commercial utility but the greater the potential for lasting environmental harm.
Why the use of untreated aliphatic-aromatic co-polyesters remains an inadequate solution requires some background on current waste disposal practices. Two primary waste disposal practices include landfill use and composting. Landfilling at its simplest involves the dumping of material that is either non-biodegradable or relatively difficult to degrade into a specially-structured pit, lined with plastic and/or clay, and covered over with dirt when full. Landfilling is also known as solid waste treatment. In landfills, solid waste is isolated from ground water and the air; the waste remains dry, is not generally subject to microbial action and therefore decomposes very slowly. Ultimately, landfill use is not a sustainable, long-term solution, given its ever-increasing economic and environmental costs. For example, even though non-biodegradable aromatic polyesters, such as PET, may be degraded chemically in a landfill, such degradation requires significant energy input in terms of high temperatures and potential cleanup of the surrounding soil and ground water owing to the harsh acids needed to break the polymer backbone.
Composting, especially of municipal waste, can be an ecologically attractive alternative to landfilling, but typically also extracts a higher cost because of the need for initial waste sorting and/or separation. See De Wilde, B. and Boelens (1998) J., Polymer Degradation and Stability, 59:7–12 describing implementation efforts in Germany, which required initial source-separated waste collection. Composting involves the biodegradation of materials by microbial action. It is largely a two-step enzymatic process that occurs under mildly anaerobic conditions, at temperatures typically not higher than 70° C. and averaging near 55°–60° C., at 100 percent relative humidity and during an exposure period from several weeks to several months. The first step of polymer biodegradation in compost occurs via hydrolysis of the polyester backbone accomplished by extra-cellular hydrolytic enzymes. These are normally secreted by a mixture of microbial flora and cleave the polyester backbone into smaller polymer fragments and/or the respective monomers. Eventually, cleavage results in fragments and/or monomers capable of being taken up by one or more of the microbial flora, which may or not be the same as those that secreted the enzymes. The second step is biological: microbes take up the fragments and/or monomers and metabolize them into biomass, biogas which includes CO2 and liquid leachate.
Besides serving as an alternative to landfill use, composting is also used as a landfill auxiliary to reduce the amount of non-recyclable, solid landfill waste and to produce cheap, beneficial fertilizer. One limitation to marketing compost, however, is its visible contamination by un-degraded biodegradable polymers, such as plastic film and fiber fragments. Because composting is an environmentally attractive waste process that can result in a commercially valuable product, there is a need for a method that improves the degradation of biodegradable polymers, particularly aliphatic-aromatic co-polyesters, during composting. Such a method would augment the environmental and commercial benefits of composting by reducing the need for and cost of initial source separation of waste.
Other waste disposal practices include wastewater treatment systems, septic systems and garbage disposal systems. As with landfilling and composting, wastewater treatment is practiced at the level of the community and involves the treatment of sewage and the renovation of wastewater before the water is re-used or re-enters a body of water. Preliminary treatment of sewage screens out solid material, such as diapers and other co-polyester goods, sand, gravel, large food particles from garbage disposals, etc., from untreated human waste and grey water. The collected debris is subsequently disposed of in a landfill and/or composting system. Other kinds of sedimented organic waste collected from wastewater systems include sludge and scum. These may contain co-polyesters that may be biodegraded. Once sedimented from wastewater, sludge and scum are pumped into digesting tanks where microorganisms break down the waste for about a month, which is then sent to a landfill. Septic systems and garbage disposal systems are practiced at the individual residence level and are typically not implicated in the disposal of goods made of high aromatic co-polyesters, like soda bottles or diapers. Disposal of such goods in these systems generally causes costly damage.
Certain aliphatic-aromatic co-polyesters are biodegradable under mildly anaerobic composting conditions and yet retain some desirable material properties, such as gas barrier permeability, strength, chemical resistance, and sterility (Muller et al., supra). Specifically, aliphatic-aromatic co-polyesters having up to 60 mol per-cent aromatic acid relative to the total acid content have been reported biodegradable. See Gouda et al. (2002) Biotechnol Prog 18:927–934; Witt et al. (1999) Angew Chem Int Ed 38(10):1438–1442; Kleeberg et al. (1998) Appl. Env. Microbiol. 64(5):1731–1735; Muller et al. (supra); and U.S. Pat. No. 6,255,451 to Koch et al. These co-polyesters are therefore eminently compostable. On the other hand, high aromatic content co-polyesters, i.e., those having greater 60 mol percent aromatic acid relative to the total acid content, are generally considered to have a biodegradation rate “so small that such materials will not be suitable for degradation in a composting process” (Muller et al., supra).
Various methods to increase the degradation rates of aliphatic-aromatic co-polyesters using hydrolytic enzymes and/or various microbial consortia to mineralize the polymers into carbon dioxide, biomass, and liquid leachate have been reported. These reports include: the treatment of foodstuff waste using hydrolytic enzymes, e.g. lipases, esterases, proteases, amylases, cutinases, etc. (U.S. Pat. No. 6,350,607 to Cooney, Jr.); the use of enzyme catalysts and/or mixed cultures of microorganisms for the degradation of polyesters (U.S. Pat. Nos. 5,990,266 and 6,191,176 to Tadros et al.); Abou-Zeid et al., supra; Witt et al., supra; Muller et al., supra; Kint and Munoz-Guerra, supra; Kleeberg et al., supra; U.S. Pat. No. 6,255,451 to Koch et al.; and Gouda et al., supra. However, these reports used copolymers having an aromatic acid content of not greater than 60 mol percent of the total acid content. None of these reports discuss methods that have effectively degraded high aromatic content co-polyester.
The technical problem of composting aliphatic-aromatic co-polyesters having greater than 60 mol percent aromatic acid relative to the total acid content may be exemplified by the obstacles in biodegrading polyethyleneterephthalate (PET). In theory, PET can be subjected to the second step of enzymatic action in composting and therefore biodegraded. That is, the microbial flora may catabolize monomers that comprise the aromatic polyesters of PET. Indeed, microbes that mineralize terephthalic acid have been isolated from a variety of environments. See Bramucci et al. (2002) Appl Microbiol Biotechnol, 58:255–259 and Junker, F. and Cook, A. (1997) Appl Environ Microbiol 63:2403–10. However, the difficulty in composting PET lies, not with the second, but with the first step of enzymatic action; that is, the breakdown of the polymer backbone through microbe-produced hydrolytic enzymes, resulting in cleaved fragments and monomers that microbes can mineralize. Since PET in essence comprises 100% aromatic diacid (terephthalic acid and esters thereof) based on total acid content, the PET polymer backbone is resistant to hydrolytic cleavage.
Modification of the polymer backbone to increase its susceptibility to hydrolytic cleavage is an important key to enhancing the breakdown of an aliphatic-aromatic co-polyester containing greater than 60 mol percent terephthalic acid, or other aromatic acid, relative to the total acid. Modification may occur in several ways. One strategy is to incorporate molecules into the polyester backbone, which may influence biodegradability, such as 5-sulphoisophthalate, into the polyester backbone. See U.S. Pat. No. 6,368,710 to Hayes. This modification makes the polymer less resistant to hydrolysis due to the activating effect exerted by the strong electron-withdrawing substitute. See Kint, D., and Munoz-Guerra, S., supra. Even though the 5-sulfoisophthalate-containing aliphatic-aromatic co-polyesters have been shown to be compostable, there remains a need for a method to increase the rate of degradation for any sulfonated aliphatic-aromatic co-polyesters having more than 60 mol percent aromatic acid content based on total acid content.
A second strategy for increasing susceptibility to hydrolytic cleavage is to treat high aromatic co-polyesters with hydrolytic enzymes before or after the co-polyesters enter the waste cycle. Numerous enzymes, known in the art, can degrade polymers containing hydrolyzable groups, such as esters, amides, etc. U.S. Pat. No. 6,255,451 to Koch et al. describes the use of a cutinase from Humicola insolens and lipases from Aspergillus niger, Mucor Miehei (Lipozyme 20,000 L), and Candida antartica (lipase component B) to degrade substrate polymers that are aliphatic polyesters, aromatic polyester amides or partially aromatic polyester urethanes. U.S. Pat. No. 6,066,494 to Hsich et al. and U.S. Pat. No. 6,254,645 to Kellis et al. describe the use of lipases or polyesterases to modify polyester fiber to enhance wettability and absorbancy of textiles. U.S. Pat. No. 6,350,607 to Cooney, Jr. discusses the use of enzymes for treatment of macerated food waste products in conjunction with garbage disposal apparatus and U.S. Pat. No. 5,464,766 to Bruno reports waste treatment compositions containing bacteria and enzymes for municipal and yard waste.
However, none of these reports concerns the breakdown of aliphatic-aromatic polymers having greater than 60 mol percent aromatic acid content relative to the total acid content. Despite the current inability to biodegrade high aromatic co-polyesters in typical waste contexts, and particularly in compost, finding a solution to biodegrade these is precisely what is needed to make municipal composting a workable waste process. Such a solution would eliminate the need for source separation of waste and provide commercially valuable fertilizer-quality compost. In addition, such a solution would also accelerate the rate of degradation of high aromatic goods disposed of in landfills.
Thus, the problem to be solved is the development of a method to increase the rate of biodegradation in typical composting conditions, and in other waste contexts, of aliphatic-aromatic co-polyesters having more than 60 mol percent aromatic acid content relative to the total acid content. The solution provided is effective enzymatic treatment of throwaway co-polyesters that begins and hastens biodegradation either before or after the co-polyesters are disposed. Put differently, the solution is an effective enzymatic treatment of throwaway co-polyesters previously regarded as very slow to biodegrade, which induces and accelerates the first step of biodegradation, i.e., hydrolytic breakdown of the polymer backbone. The present solution is especially useful for accelerating the degradation of sulfonated co-polyesters having more than 60 mol percent aromatic acid content.